1. Field of the Invention
The present invention relates to a brushless DC motor, and more particularly, to a brushless DC motor capable of enhancing the efficiency thereof by preventing leakage of magnetic flux caused by a vortex of magnetic flux generated between the permanent magnets adjacent to each other.
2. Prior Art
A brushless DC motor has a stator having excitation coils, and a rotor having permanent magnets. Torque is generated by the cooperation of the magnetic force generated at the stator by the excitation coils and the magnetic force of the permanent magnets, and the rotor is rotated by the torque.
The permanent magnets in the rotor are assembled with the rotor core in a variety of manners. FIG. 3 shows an example of the stator and the rotor used in a conventional brushless DC motor.
The stator 30 is comprised of a hollow stator core 10 having a plurality of protrusions 11 protruding inward, and excitation coils 12 wound around the protrusions 11 of the stator core 10. The rotor 20 is disposed at the inner space of the stator 30, and is comprised of a cylindrical rotor core 1 and a plurality of permanent magnets 2 inserted therein. The permanent magnets 2 are disposed along the circumferential direction of the rotor core 1 by an equal angular distance with equal angle and equal distance. The adjacent permanent magnets 2 are disposed so that the respective poles N and S thereof face to each other.
When the excitation coils 12 of the stator 30 are supplied with current, a magnetic field is generated on the protrusion part 11 of the stator core 10 by the excitation coils 12. Such a magnetic field forms a rotary magnetic field 21 together with a magnetic field of the permanent magnets 2 in the rotor 20. The rotary magnetic field 21 forms a path circulating from the pole N of one permanent magnet 2 to the pole S of another permanent magnet 2, via the protrusion 11 excited to the pole S, the inner side of the stator core 10, and the protrusion 11 excited to the pole N, successively. The torque for rotating the rotor 20 is generated by the rotary magnetic field 21.
However, since the pole of one permanent magnet 2 in the inside of the rotator core 1 is different from that of another adjacent permanent magnet 2, the magnetic flux generated between the adjacent pars of the respective permanent magnets 2 is not included in the rotary magnetic field 21, but forms a vortex 25 of the magnetic flux flowing directly toward an adjacent permanent magnet 2. Such a vortex 25 of the magnetic flux comes a leakage of the magnetic flux because it is not included in the rotary magnetic field 21 contributing to the generation of torque of the rotor 20, which may cause lowering of the efficiency of the motor.